The present invention relates generally to impact printing devices and, more particularly, to dot matrix printers generally employing solenoid type actuators.
A typical print head for a dot matrix type printer has a plurality of print wires, each actuated by an individual print wire solenoid. This type of print wire solenoid actuator is disclosed, for example, in U.S. Pat. No. 3,775,700 issued Aug. 28, 1973. The solenoid armature, which is connected to the print wire, moves axially of the print wire between a normal rest position in which the print wire is spaced from the target surface and a printing position in which the print wire is impacted upon a target surface. When the solenoid coil is energized the armature is accelerated forwardly to drive the print wire toward its printing position.
Present impact printers are designed to quickly accelerate the print wire forward to a desired velocity and to impact the target with that velocity. Thus, the actuator applies to the coil only enough energy to accelerate the print wire to the desired impact velocity. Typically, a pulse of power is applied to the coil for a limited time sufficient to insure that the desired velocity will be reached. In this regard, velocity can be affected by a number of variables such as friction losses, primarily due to variations in the tightness of wires in the wire guides, damping losses caused by viscous damping due to ink accumulation (sometimes referred to as morning sickness), and power pulse variations in part due to circuit tolerances. Present day actuator drives must be provided with a power pulse large enough to meet minimum requirements for the worst case conditions. Since these conditions are not always present, the impact velocity of the print wire is subject to considerable variation, resulting in variable print quality.
In a dot matrix type printer, the print wire will first make contact with the ribbon, pushing the ribbon against the target surface and platen or striker bar. The print wire applies maximum impact force when the wire velocity reaches zero. All the kinetic energy of the moving printing assembly is then converted to potential energy stored in the combined spring constant of the print wire as compression and deflection forces, of the armature as bending, of the paper as compression forces, and of the spring biasing means.
The combination of spring forces causes the print wire to recoil, converting the potential energy back into kinetic energy as the print wire accelerates back to its starting position. However, the return velocity due to the recoil action in conventional print wire actuators will only be 50 to 70 percent of the forward velocity, since friction losses during impact are considerable and convert some of the energy to heat. When the print wire reaches its rest position it impacts against a back stop. Because the print wire has considerable return velocity, it tends to repeatedly bounce off the back stop. This significantly increases the time before the print wire comes completely to rest. The combination of reduced return velocity and long damping time significantly increases the minimum print cycle time of the print wire.